According to Martin Marty, a professor of church history at the University of Chicago, Psalm 88 is “a wintry landscape of unrelieved bleakness.”
Psalm 88 ends by saying:
You have taken my companions and loved ones from me; the darkness is my closest friend. (Psalm 88:18, NIV).
Indeed, in Hebrew, the last word of the Psalm 88 is "darkness".
- The Artscroll Tehillim page 329